Page 17 - July 2023

Let’s start with the rigging discussion and first review the three-blade model 200 graph (right).
This particular graph presents numbers applicable to a three- blade model 200 and except for the numbers, it applies to all PT6- powered King Airs. The later models have the Ground Fine stop between Beta and Reverse and do not have the red stripes. The (+) and (-) symbols represent the areas where positive or negative thrust occurs, statically on the ramp.
As the graph shows, the position of the power lever controls two different things: Compressor Speed (N1 or Ng) and the position of the propeller’s Low Pitch Stop (LPS). (This stop also goes by the name of “Flight Idle Stop” in some references, including portions of the maintenance manuals. I have always believed that “Low Pitch Stop” is a more obvious term that better describes exactly what is being repositioned.)
Notice the flat portion of the upper line, the Beta area. This flat portion, in which N1 should not change, is appropriately known as the “Dead Band” since movement of the power lever within this range causes no response – dead reaction – from the Fuel Control Unit (FCU). By definition, the Beta Range is where the propeller’s LPS is being repositioned to flatter blade angles while N1 is not changing.
Behind Beta is the Reverse Range. By definition, not only is the LPS continuing to be repositioned to lesser blade angles – it is, in fact, going to negative angles, meaning that the propeller is pushing air forward instead of aft – but also N1 is proportionally increasing, getting greater the more aft the power lever is moved. Typically, Maximum Reverse, all the way aft, should yield an N1 speed of about 85%. Remember that the relative speed of the compressor is not the same as the engine’s relative power output.
9 17 30
9 17 30
JULY 2023
KING AIR MAGAZINE • 15